The present invention relates to making OLED devices.
In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media are required to produce the RGB pixels. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electronic transport layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow-mask method is that the mask holes can become plugged over time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
A suitable method for patterning high-resolution OLED displays has been disclosed in commonly-assigned U.S. Pat. No. 5,851,709 by Grande et al. this method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable color-forming organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow-mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor element and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Littman and Tang (commonly-assigned U.S. Pat. No. 5,688,551) teach the patternwise transfer of organic EL material from an unpatterned donor sheet to an EL substrate. A series of patents by Wolk et al (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teaches a method that can transfer the luminescent layer of an EL device from a donor element to a substrate by heating selected portions of the donor with a laser beam.
In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material is deposited on a substrate in a selected pattern via the use of a donor coating on a support and an aperture mask. The aperture mask can be a separate entity between the donor layer and substrate (as in FIG. 1 in the aforementioned patent), or can be incorporated into the donor layer (as in FIGS. 4, 5, and 6 in the aforementioned patent).
The EL material transfer is preferably done under conditions of reduced oxygen and/or water, using a chamber such as Tang describes in the aforementioned patent. The donor layer (and aperture, if separate) and substrate must be kept in close proximity. As an example, Tang shows an aperture or donor layer held close to or on a passivating layer, such that there is a preferable distance between the donor layer and the intended donor target of the bottom electrode. The use of vacuum or reduced pressure can facilitate the transfer of the EL material from the donor to the substrate. The use of such conditions during transfer is also advantageous in that some EL materials are sensitive to oxygen and/or moisture. For example, aluminum hydroxyquinoline (Alq), which is used in OLED devices, is known to react with water. In addition, the electrode materials used on both small molecule and polymer EL devices are extremely unstable in air. The use of low oxygen and/or water conditions during the transfer step can help reduce the failure rate of OLED devices. Additionally, losses of OLED devices can occur because of degradation of the donor material before transfer to the substrate in the methods taught by Littman, Tang, and Wolk. The donor material is generally transported from its preparation site to the site where it is transferred to the substrate. Contamination of the donor by oxygen, moisture, and/or other atmospheric components is possible during this time. This can lead to reduced yields of OLED displays prepared from the donor.
It is therefore an object of the present invention to provide a method, which eliminates the need for a shadow mask.
It is a further object of the present invention to provide a method which uses a donor element but eliminates problems associated with providing a donor element at a remote location from where it is to be used and shipping such donor element without causing contamination or damage to the donor element.
It is a further object of the present invention to provide an improved shadow mask free method which effectively can be used to produce full color OLED displays.
This object is achieved by an in-situ method for fabricating, at least in part, an OLED device that is moisture- or oxygen-sensitive, such method comprising the steps of:
a) providing into a controlled atmosphere coater a receiver element which will form part of the OLED device;
b) providing into the controlled atmosphere coater a donor support element and coating such donor support element to produce a donor element with one or more layers required to produce all or part of the OLED device;
c) controlling the atmosphere in the controlled atmosphere coater so that either the water vapor level is less than 1000 ppm but greater 0 ppm or the oxygen level is less than 1000 ppm but greater than 0 ppm, or both the water vapor level and oxygen level are respectively less than 1000 ppm but greater than 0 ppm;
d) positioning the coated side of the donor element in material transferring relationship to the receiver element to be coated; and
e) applying radiation to the donor element to selectively transfer one or more layers from the donor element to the receiver element.
An advantage of the method described in this invention is that it is useful in producing OLED devices without introducing moisture, oxygen, or other atmospheric components and without using shadow mask.
In accordance with the present invention, donor element is made with the same controlled-atmosphere coater used for transferring materials from the donor element to the OLED receiver element. This provides a number of advantages including:
it reduces the need for donor storage and transport, and concomitant contamination;
it can reduce or eliminate damage and contamination from the contact of the donor side with the support side of the donor;
it reduces the keeping requirements for the donor; and
it can improve the yield of OLED devices.
A further advantage is that this method can be fully automated including donor and substrate media handling. The present invention is particularly suitable for forming organic layers over a large area having a number of OLED display devices, which are in the process of being formed, thereby increasing throughput.
A further advantage is that the donor element can be cleaned and reused.
A further advantage (relative to the vacuum-based technique) is that added techniques can be used for coating, including solvent-based coating such as spin coating, curtain coating, spray coating, Gravure-wheel coating, and others). Additional techniques can be used for facilitating the material transfer, such as vacuum hold-down. Additional techniques can be used for donor cleaning, such as solvent cleaning. It is also easier to place a radiation source in such an environment. It is also easier to provide a vacuum-based positioning of the donor on the substrate.